to become upset, usually out of proportion to the situation.
You: "C'mon, hurry up, I like to arrive at the movies at least 30 minutes early so I can Scotchgard my seat!"
Your friend: "Dude, don't get your skivvies in a bunch!"
You post a detailed analysis of a Star Trek episode, and someone leaves a disagreeing comment, causing you to write a 10,000-word essay defending your expertise. Someone then comments "Don't get your skivvies in a bunch."